Abstract

We have monitored photometrically the Y0 brown dwarf WISEP J173835.52+273258.9 (W1738) at both near- and mid-infrared wavelengths. This ≾ 1 Gyr old 400 K dwarf is at a distance of 8 pc and has a mass around 5 M Jupiter. We observed W1738 using two near-infrared filters at λ ≈ 1 μm, Y and J, on Gemini Observatory and two mid-infrared filters at λ ≈ 4 μm, [3.6] and [4.5], on the Spitzer observatory. Twenty-four hours were spent on the source by Spitzer on each of 2013 June 30 and October 30 UT. Between these observations, around 5 hr were spent on the source by Gemini on each of 2013 July 17 and August 23 UT. The mid-infrared light curves show significant evolution between the two observations separated by 4 months. We find that a double sinusoid can be fit to the [4.5] data, where one sinusoid has a period of 6.0 ± 0.1 hr and the other a period of 3.0 ± 0.1 hr. The near-infrared observations suggest variability with a ~3.0 hr period, although only at a ≾ 2σ confidence level. We interpret our results as showing that the Y dwarf has a 6.0 ± 0.1 hr rotation period, with one or more large-scale surface features being the source of variability. The peak-to-peak amplitude of the light curve at [4.5] is 3%. The amplitude of the near-infrared variability, if real, may be as high as 5%–30%. Intriguingly, this size of variability and the wavelength dependence can be reproduced by atmospheric models that include patchy KCl and Na_2S clouds and associated small changes in surface temperature. The small number of large features, as well as the timescale for evolution of the features, is very similar to what is seen in the atmospheres of the solar system gas giants.